1. Field of the Invention
The present invention relates to electrochromic devices which modulate the transmittance of light entering a window depending on the incident solar intensity and, in particular, relates to electrochromic devices which are self-powered to modulate the transmittance of light entering a window without needing external power supplies.
2. Description of the Prior Art
Presently there exists in excess of 19 billion square feet of windows in commercial and residential buildings, and more than 600 million square feet of windows are being installed in new buildings throughout the United States each year, culminating in enormous cost and energy demands for air conditioning to negate the unwanted solar heat gains through these windows. Sometimes, static solar gain control coatings are applied to windows. This practice is disadvantageous, however, in that sunlight is blocked even when it is needed for lighting and heating during cold weather.
There have been some apparatus and methods developed for controlling the transmittance of light through window panes. For example, U.S. Pat. No. 4,768,865, discloses a conventional electrochromics film on a window that is responsive to the application of an electric field to change from a high-transmittance, non-absorbing state to a lower-transmittance, absorbing or reflecting state.
U.S. Pat. No. 4,958,917, discloses the use of an electrochromic coating for dimming automobile windows, including a plurality of solid electrochromic elements arranged in a horizontal abutting fashion and adhered to the glass window, each element being controlled independently of the others.
U.S. Pat. No. 4,958,917, discloses a specialized combination of two electrochromic cells which, together, are capable of reducing the transmittance of visible light to less than the usual lower limit of 7% for known electrochromic coatings.
While U.S. Pat. Nos. 4,768,865, and 4,958,917 patents teach electrochromic coatings or controlling the transmittance of light through window panes, these coatings require external power supplies and, to be practical, the need for wiring into a building""s electrical system.
U.S. Pat. No. 4,475,031 discloses a self-contained sun-sensitive window made up of liquid nematic crystals (LC), sandwiched between two transparent sheets and powered directly by a solar cell. Liquid nematic crystals, however, are not effective,in blocking heat radiation. Rather, they merely scatter light, thus making a window translucent, but not effectively blocking heat gain from the sun""s rays. A further disadvantage of such liquid nematic crystal technology is that a constant source of energy is required to change the translucence of the window, thus requiring a substantial and continuous source of electric power as long as transparency instead of translucence is desired.
U.S. Pat. No. 5,384,653, assigned to the same assignee as the present application, describes a stand-alone photovoltaic powered electrochromic window. The window of this patent includes an electrochromic material that has been deposited on one pane of the window in conjunction with an array of photovoltaic cells deposited along an edge of the pane to produce the required electric power necessary to vary the effective transmittance of the window. While providing a self-powered electrochromic window, the window of this patent requires increased layers in the electrochromic material in addition to using a portion of the window space for the photovoltaic array along the periphery of the window. Use of only the periphery of the window for the photovoltaic cells could result in the sunlight entering the building without actually contacting the photovoltaic cells.
U.S. Pat. No. 5,377,037, assigned to the same assignee as the present application, describes an electrochromic-photovoltaic film for light-sensitive-control of optical transmittance. The variable transmittance optical component of this patent includes an electrochromic material and a photovoltaic device-type thin film solar cell deposited in a tandem type, monolithic single coating over the component. A resistor is connected in series across the electrochromic material and photovoltaic device controlling the activation and deactivation of the electrochromic material. The electrical conductivity between the electrochromic material and the photovoltaic device is enhanced by interposing a transparent electrically conductive layer.
In U.S. Pat. No. 5,377,037 an electrical connection exists between the electrochromic material and the photovoltaic device. While successfully providing a variable transmittance optical component that does not require an external power supply, the optical component of this patent requires at least eight thin layers to operate and is very complicated to construct due to the thin layers and delicacy of manufacture.
U.S. Pat. No. 5,604,626 of Teowee et al. disclose a user controllable photochromic device (UCPC) having the means for controlling the electrical connection between a first conducting electrode and a second conducting electrode, as shown in FIG. 6 and described in column 14, lines 1-21. The photochromic device of this patent is not spontaneously bleached upon removal of light, but instead uses an external bleach voltage. Teowee et al. does state at column 13, lines 64-67 that, xe2x80x9cOf course, an external bleach voltage is optional especially for any devices which may rapidly self bleach when the radiation source is removed.xe2x80x9d
However, Teowee et al. fails to provide a positive disclosure or teaching of any device design that may rapidly self bleach upon removal of a radiation source, and only positively teach user controllable photochromic (UCPC) devices that specifically rely upon an external power source for user control of both coloration and bleaching (column 14, lines 1-22).
Regardless of the previous attempts to reduce the amount of solar energy entering a window, there remains a need for a self-darkening window coating which modulates the transmittance of incoming light depending on the incident solar intensity without utilizing valuable window space. Furthermore, the need remains for a self-darkening window which modulates depending on the amount of light entering the window and not the amount of light striking the periphery of the window.
There is a need in the art of photochromic devices for a self-powered photoelectro chemical-electrochromic device design that automatically modulates transmittance of in coming light and that spontaneously bleaches upon removal of light without an external power source.
The present invention is a photoelectrochemical-electrochromic device. The device comprises a first transparent electrode and a second transparent electrode in a parallel, spaced relation to each other. The first transparent electrode is electrically connected to the second transparent electrode.
The device of the present invention further comprises an electrochromic material applied to the first transparent electrode and a semiconductor coating applied to the second transparent electrode. An electrolyte layer contacts the electrochromic materials and the semiconductor coating. The electrolyte layer has a redox couple whereby application of light, the semiconductor coating absorbs the light and the redox couple oxidizes producing an electric field across the device modulating the effective light transmittance through the device.
In a preferred embodiment, the device further comprises a first transparent substrate and a second transparent substrate. The first transparent electrode is applied to the first transparent substrate and the second transparent electrode is applied to the second transparent substrate. Preferably, the first and second transparent substrate comprises a material selected from the group consisting of glass and plastic.
In another embodiment of the device of the present application, the electrochromic material preferably comprises a material selected from the group consisting of WO3, IrOx, V2O5, and NiO and the semiconductor coating layer preferably comprises a porous material of metal oxide selected from the group consisting of TiO2, CdS, ZnS, ZnO, and WO3. Furthermore, the device of present invention preferably comprises a layer of dye applied on the surface and into the pores of the semiconductor coating layer. The dye preferably comprises transition metal complexes selected from the group consisting of ML3 and ML2X2 wherein M is selected from the group consisting of Fe, Ru, Os, Cr, Mo and Mn, L comprises bipyridine-type ligand selected from the group consisting of 2,2xe2x80x2-bipyridine-4,4xe2x80x2-dicarboxylic acid and 4,4xe2x80x2-dimethyl-2,2xe2x80x2-bipyridine, and X comprises a monodentate ligand selected from the group consisting of CN, SCN, Cl, Br, I and H2O; poryphyrins comprising zinc tetra (4-carboxyphenyl) porphyrin; phthalocyanines comprising 4,4xe2x80x2,4xe2x80x34xe2x80x2xe2x80x3-tetraazaphthalocyanine; and perylenes comprising 3,4,9,10-perylene tetracarboxylic acid.
In yet another preferred embodiment, the electrolyte layer has a predetermined thickness to provide high lateral ionic conductivity to darken the entire immediately adjacent electrochromic material. When the electrolyte layer""s thickness is less than the predetermined thickness, high lateral conductivity does not occur and only those portions of the electrochromic material adjacent the electrolyte exposed to the light will actually darken.
The present invention is also a method of modulating the transmittance of light. The method comprises electrically connecting a first transparent electrode and a second transparent electrode in parallel, spaced relation to each other. Next, an electrochromic material is applied to the first transparent electrode and a semiconductor coating is applied to the second transparent electrode. Then, an electrolyte layer is positioned to contact the electrochromic material and the semiconductor coating. The electrolyte layer has a redox couple, whereby the first and second electrodes, the electrochromic material, the semiconductor coating, and the electrolyte layer form a photoelectrochemical-electrochromic device. Finally, the photoelectrochemical-electrochromic device is exposed to a light source wherein the semiconductor coating and/or the sensitizing dye absorbs the light and the redox couple becomes oxidized producing an electric field across the photoelectrochemical-electrochromic device to vary the effective light transmittance through the photoelectrochemical-electrochromic device.
In a preferred embodiment, the method further comprises applying the first transparent electrode to the first transparent substrate and applying the second transparent electrode to the second transparent substrate. Also, preferably, the light absorbing and transferring means comprises a dye substance, e.g., a monomolecular chromophore dye applied to the semiconductor layer. In yet another embodiment, the electrolyte layer has predetermined thickness to provide high lateral ionic conductivity.